1. Field of the Invention
The present invention relates to an optical disk drive for reading and/or writing data from/on an optical disk.
2. Description of the Related Art
In optical disk technologies, data or information can be read out from a rotating optical disk by irradiating the disk with a relatively weak light beam with a constant intensity, and detecting the light that has been modulated by, and reflected from, the optical disk.
On a rewritable optical disk such as a DVD-RAM, a recording material film, from/on which data can be read and written optically, is deposited by an evaporation process, for example, on the surface of a substrate on which tracks with spiral lands or grooves are arranged. In writing data on a DVD-RAM, the optical disk is irradiated with a light beam, of which the optical power has been changed according to the data to be written, thereby locally changing the property of the recording material film. A portion of the recording material film, which has been irradiated with the light beam, comes to have a different refractive index from that of the other portions that have not been irradiated with the light beam. Such a portion with the varied refractive index will be referred to herein as a “recording mark”, while an interval between two adjacent recording marks on the same track will be referred to herein as a “space”. By adjusting the lengths of these recording marks and spaces, user data can be written on the tracks.
It should be noted that the level difference between the lands and grooves and the thickness of the recording material film are smaller than the thickness of the optical disk substrate. For that reason, those portions of the optical disk, where data is stored, define a two-dimensional plane, which is sometimes called an “information storage plane”. However, considering that such an “information storage plane” has a physical dimension in the depth direction, too, the term “information storage plane” will be replaced herein by another term “information storage layer”. Every optical disk has at least one such information storage layer. Optionally, a single information storage layer may actually include a plurality of layers such as a phase-changeable material layer and a reflective layer.
FIGS. 1A through 1D illustrate the configuration of a DVD-RAM. Specifically, FIG. 1D schematically illustrates the track arrangement of an optical disk 100 manufactured to the specifications of a DVD-RAM. FIG. 1B is a plan view showing the area inside the dashed line in FIG. 1D on a larger scale. FIG. 1A is a cross-sectional view illustrating a half of the area shown in FIG. 1B on the left-hand side of the arrow A, while FIG. 1C is a cross-sectional view illustrating the other half of the area shown in FIG. 1B on the right-hand side of the arrow A.
As shown in FIGS. 1A through 1D, a spiral string of tracks, where land tracks and groove tracks are alternated with each other, is provided on the substrate surface of the optical disk 100. Each of those tracks has a length corresponding to one round of the optical disk. At the position pointed by the arrow A in FIG. 1B, a groove track changes into a land track, and a land track changes into a groove track. Although not shown, an information storage layer and a protective coating have actually been deposited on the substrate surface with such level differences.
To read or write data from/on the optical disk 100, the light beam always needs to maintain a predetermined converging state on a target track on an information storage layer. For that purpose, a “focus control” and a “tracking control” are required. The “focus control” means controlling the position of an objective lens perpendicularly to the information storage layer (which direction will be referred to herein as a “substrate depth direction”) such that the focus position (or converging point) of the light beam is always located on the information storage layer. On the other hand, the “tracking control” means controlling the position of the objective lens along the radius of a given optical disk (which direction will be referred to herein as a “disk radial direction”) such that the light beam spot is always located right on a target track.
Suppose a light beam has been converged by an objective lens to form a beam spot on the information storage layer of the optical disk 100. More specifically, the beam spot is supposed to have been formed at the point indicated by the solid circle in FIG. 1B and the optical disk 100 is supposed to be spinning counterclockwise. In that case, as the optical disk 100 turns, the beam spot shown in FIG. 1B moves along the land track in the direction pointed by the arrow B1. And when the optical disk 100 has made one round, the light beam spot on the land track will return to the area inside the dashed line in FIG. 1B as pointed by the arrow B2. At this point in time, the beam spot is located closer to the outer edge (or the lead-out area) of the optical disk 100. As the optical disk 100 further turns, the beam spot soon passes the boundary pointed by the arrow A in FIG. 1B and moves rightward as pointed by the arrow C1. When passing the boundary, the beam spot hops from the land track to a groove track. And when the optical disk 100 has made another round, the beam spot that has been moving along the groove track as pointed by the arrow C2 will go over the boundary pointed by the arrow A. Then, the beam spot will hop from the groove track onto a land track.
In this manner, in the optical disk 100 manufactured to the specifications of a DVD-RAM, data is read and written from/on both land tracks and groove tracks. On the optical disk 100, the alternated land and groove tracks are arranged spirally and the changing point between the land and groove tracks is defined by the boundary pointed by the arrow A.
As described above, every time the optical disk 100 makes one round, the beam spot shifts one track closer to the outer edge of the disk. That is why to keep the beam spot located on the same track for a long time while turning the optical disk 100, every time the optical disk 100 makes one round, the beam spot location needs to be shifted one track closer to the inner edge of the disk. Such an operation is called a “still jump” or a “retrace jump”.
FIG. 2A is a cross-sectional view schematically illustrating how a light beam has been converged by an objective lens on a land track of the optical disk 100. FIG. 2B is a plan view illustrating a portion of land/groove tracks. As shown in FIG. 2B, the land/groove tracks wobble in a predetermined period. Such wobbling modulates the intensity of a read signal in a relatively long period. When a low pass filter extracts a periodic variation caused by the wobbling from the read signal, a wobble detection signal can be generated. The period of the wobble detection signal corresponds to the wobbling period of the land/groove tracks on the optical disk 100, and may be used as a reference for a clock signal. More particularly, the wobble of the tracks on the optical disk 100 is defined such that frequency and phase are fixed with respect to header data. Accordingly, if a phase-locked loop (PLL) control is carried out, and the oscillation frequency of a voltage controlled oscillator (VCO) is regulated, by reference to the wobble detection signal, a reference signal can be generated as a timing signal that is required to read the header data. An optical disk with such wobbling tracks is disclosed in Japanese Patent Application Laid-Open Publications Nos. 2001-266358 and 11-296911.
Due to the difference in cross-sectional shape as shown in FIG. 2A, a land track and a groove track exhibit mutually different read/write characteristics. Thus, in the prior art, to resolve such a difference in the read/write characteristics exhibited by land and groove tracks, the ratio of the width of a land track to that of a groove track (which will be referred to herein as an “L/G ratio”) is sometimes set to be not equal to one.
FIG. 3A is a plan view illustrating where recording marks are left on a land track when the L/G ratio is one. FIG. 3B is a cross-sectional view illustrating which portion of the track a light beam hits in such a situation. And FIG. 3C shows the waveform of a wobble signal that has been read from a land track with such recording marks. On the other hand, FIG. 4A is a plan view illustrating where recording marks are left on a land track when the L/G ratio is smaller than one. FIG. 4B is a cross-sectional view illustrating which portion of the track a light beam hits in such a situation. And FIG. 4C shows the waveform of a wobble signal that has been read from a land track with such recording marks.
If the L/G ratio is smaller than one (i.e., if land tracks are narrower than groove tracks), some recording mark may be left partially outside of an edge of the land track as shown in FIGS. 4A and 4B due to an off track. In that case, noise is created in the wobble signal as shown in FIG. 4C owing to the presence of such a recording mark and the wobble signal cannot be detected with good stability anymore. Then, the PLL cannot be locked as intended, which is a problem. Also, even if the PLL can be locked anyway, it should take a longer time to achieve the phase locking. Therefore, even if the PLL has been locked successfully once, the light beam spot may have gone over the target track more often than not. In such a situation, it is necessary to try to get the PLL locked all over again (which is sometimes called a “retrace operation”), thus preventing a series of data read and write operations from being started normally. Suppose the target track, from/on which data should start being read or written, is the land track on which the light beam spot is located as shown in FIG. 1B. Even if the drive starts trying to get the PLL locked on this target track, the PLL may not get locked on the same target track for the reasons described above. Then, to carry out a retrace operation, the beam spot that has moved in the direction pointed by the arrow B2 needs to jump one track back toward the inner edge of the disk such that the drive can retry to get the PLL locked on the target track. However, the target track in problem, on which the PLL could not get locked last time, may be unqualified to get the PLL locked due to the low quality of the wobble signal. In that case, the PLL still cannot be locked no matter how many times the drive retries to get the PLL locked.